Long-distance and large-capacity optical communication using optical amplification technique has a problem of degradation in transmission characteristics due to optical nonlinear phenomena in single mode fibers.
The optical nonlinearity in a single mode fiber varies in proportion to a nonlinear coefficient n2/Aeff obtained by dividing a nonlinear refractive index n2 by an effective cross-sectional area Aeff (G. P. Agrawal, “Nonlinear Fiber Optics (second edition)”, Academic Press, 1995, particularly refer to section 2.3.1, p. 42). Accordingly, the degradation in the transmission characteristics due to the optical nonlinear phenomena in the long-distance and large-capacity optical communication can be reduced by decreasing the nonlinear coefficient in the single mode fiber by increasing the effective cross-sectional area Aeff of the single mode fiber.
Thus, as for the conventional single mode fibers, attempts have been made to increase the effective cross-sectional area Aeff in the design and optimization of the refractive index profile forming the optical waveguide structure. Up to this time, in an operating wavelength region from about 1310 nm to 1625 nm, characteristics of single mode fibers with the effective cross-sectional area Aeff of about 70 μm2 to 150 μm2 have been disclosed (see, Japanese Patent Application Laid-open No. 9-274118 (1997) (Claim 6), Japanese Patent Application Laid-open No. 11-218632 (1999) (Claim 1), Japanese Patent Application Laid-open No. 2001-033647 (Claim 1 and a representative drawing FIG. 1), and Japanese Patent Application Laid-open No. 2001-147338 (Claim 13, paragraph [0022]), for example).
On the other hand, as for conventional 1.3 μm-band zero dispersion single mode fibers, they can be implemented with a simple two layer structure including a core region with a higher refractive index and a cladding region with a lower refractive index than the core region. Since they have a comparatively large effective cross-sectional area Aeff of about 80 μm2 near the wavelength 1550 nm, they can achieve good connection and handling characteristics, and have been widely used in the optical communication and optical wiring until now.
The increase in the effective cross-sectional area Aeff in the design and optimization of the refractive index profile, however, will generally complicate the refractive index profile in the radial direction in a cross section of a single mode fiber (SMF). In addition, in the SMF whose effective cross-sectional area Aeff is increased, the optical confinement of the light propagating through the optical fiber within the optical core reduces, and the bending loss characteristic is deteriorated. This offers a problem in that the actual value of the feasible effective cross-sectional area Aeff is limited to a region in which the acceptable bending loss characteristic is achieved. For example, the bending loss at the bending radius 10 mm is limited to a range from 10 dB/m to 100 dB/m or less.
In addition, as for the SMF whose effective cross-sectional area Aeff is increased, the theoretical cutoff wavelength in the higher-order LP11 mode has a tendency to shift to a longer wavelength region. This presents a problem in that the effective operating wavelength region is limited to a longer wavelength region of 1400 nm or more, for example (refer to Japanese Patent Application Laid-open No. 2001-147338 (Claim 13 and paragraph [0022], for example).
Furthermore, although the conventional SMF has a simple structure and comparatively large effective cross-sectional area Aeff, its adaptive or applicable area is limited to a region in which the bending radius is comparatively large such as from 20 mm to 30 mm because of the degradation in the bending loss characteristics. Accordingly, it has a drawback of being unable to make compact in actual optical transmission paths or optical wiring because the wiring or storage space is limited in accordance with the acceptable bending radius. Thus, to improve the bending loss characteristic of the conventional SMF, some SMFs provided for reduction of mode field diameter (MFD) have been developed. However, the SMFs of this type have a problem of impairing the handling characteristics such as the splice loss as a result of the reduction in the MFD.